This invention relates to the art of color television camera systems and, more particularly, to improvements relating to contour enhancements for purposes of improving picture sharpness.
It is known in the art that picture sharpness may be increased by some form of aperture correction or contour correction. Such correction tends to correct for apparent loss of resolution due to the imperfections in the production and transmission of picture signals. It is known in the art for cameras using red, green and blue camera pickup devices to extract high frequency horizontal and vertical spatial information from one of the three color signals and then add this information back to each of the three color signals or the luminance signal. Generally, it is the green signal that is employed in deriving an aperture or contour correction signal since the majority of enhancement required for most average scenes is best obtained from contours out of the green video signal. This green derived contour correction signal is then added to each of the three color signals individually or to the luminance signal. In this way the thus aperture corrected color signal provides sharp transistions.
Aperture corrected color signals derived only from the green video signal exhibit problems in that scenes which contain little or no green information appear soft or lacking in sharpness. Thus, for example, large area red scenes are noticeably soft since there is no contour information from the red video signal. In such a system, then, where the red and green video signals are separated from each other, the combination of enhanced green derived contour signal and nonenhanced red provide an appearance of unbalance and will sometimes cause a viewer to conclude that the picture is coming from a camera that seems to be out of focus. Additionally, where the red and green video signals are adjacent to each other, the green enhanced contour signals may extend into the red video signal area and reduce the luminance on the edge of the red signal. If the red image is small, this low luminance becomes a greater portion of the total image size and causes the red image to appear dark.
The enhancement unbalance between separate red and green images could be cured by deriving a contour signal from the red video and summing this signal with the green contour signal. However, this may cause other problems when the two images are adjacent to each other since the transitions of the red and green contour signals may cancel at the adjacent point causing a loss or unbalance of enhancement at the color edges. Additionally, where there is some misregistration or overlap of the images, this solution may accentuate the misregistration error rather than be subdued as is the case when the contour signal is derived from only a single color video such as the green video signal.
The problem of using contour signal derived from only one of the three video color signals as well as the tendency to accentuate misregistration error has been addressed in the prior art. U.S. Pat. No. 4,209,801, to R. A. Dischert et al., presents a system wherein the aperture correction signal is normally derived from the green video signal but also from the red video signal when the red video signal attains a magnitude of essentially twice the amplitude level of the green video signal. The U.S. Pat. No. 3,681,520, to H. Schneider, presents a system wherein the aperture correction signal is derived by examining the amplitude of the green, blue and red video signals and using the signal having the greatest amplitude to derive the contour correction or aperture correction signal. A still further example of relevant prior art is the U.S. Pat. No. Re. 29,570, to Breimer et al., and which presents a system for preventing over compensation for aperture correction particularly during superposition error. The contour correction signal is derived from only one of the color signals which is then added to each of the red, green and blue color signals separately or by way of a matrix network.
From the noted prior art patents, it is seen that it is known to sharpen a picture generated by a color camera by adding an aperture correction or contour signal to the video signal to accentuate the edges thereof. However, if the contour signal is generated always from only one of the three colors, such as the green color video, then scenes which are largely of another color, such as red, result in a softness at the edges since there is no contour information from the red video signal. However, if the contour signal is derived from more than one color, such as both green and red video signals, then, other problems may occur, such as total cancellation of the contour signals when images are adjacent to each other, or accentuating registration errors of a slightly misregistered picture.
The present invention is directed to providing improvements in picture sharpness by providing contour signals in such a manner as to minimize accentuation of color registration errors. In accordance with one aspect of the present invention, this is achieved by providing contour signals derived from the green video signal and the red video signal. The two contour signals are selectively added together only at such times as the addition of the two signals would not result in accentuation of color registration errors.
Thus, it is a primary object of the present invention to provide aperture correction signals or contour signals to be applied to the video signal to accentuate the edges of the video signals in such a way that accentuation of color registration errors is minimized.
It is a still further object of the present invention to provide a contour signal derived from the red video which is then selectively added to a green video derived contour signal only when such addition would not result in accentuating color registration errors.
In accordance with the invention, a contour correction system is provided for a TV camera system which provides at least first and second video color signals, such as green and red video signals. First and second contour signals are respectively derived from the first and second video signals. Normally, the first contour signal is added to all of the video signals to improve picture sharpness. The second contour signal is selectively added to the first contour signal to provide a composite contour signal for addition to the video signals only during such times as the composite contour signal does not accentuate color misregistration.